1. Field of the Invention
This invention relates to a injection control method for an injection molding machine, in which an injection mechanism is driven by a motor, and to the injection molding apparatus.
2. Prior Art
The techinques relating to the method and apparatus of the present invention are not generally known or seen. Only patents previously invented by this inventor, which are described in the U.S. Pat. No. 4,540,359 and Japanese patent No. 59-726, No. 59-224324 describe the method or the apparatus.
According to the above mentioned patents by setting an injection speed and a torque upper limit, and by employing the feedback of the injection speed, which is electrically detected by a speed sensor, and by adopting the closed loop control so that the injection speed may become the set point, the injection speed or the injection pressure is controlled in accordance with the mutual relationship, between the set points of the injection speed and the torque value, and the load applied on the driving source. The injection pressure control is effected by driving the motor by means of setting the torque value (the current upper limit value of the motor) through the open loop.
The speed control phase is carried out with a closed loop control which senses the injection speed, from the beginning of injection to the end of a period during which material is fed into the metal mold. In order to mitigate the adverse effect of the inertia force of the motor rotor, in a second phase, the control is changed to control the injection pressure rather than motor speed. This is done by changing the set point of the torque value (current upper limit value of motor), after a retarding control in which the injection speed is almost linearly reduced.
On account of the disadvantage in the characteristics of motor, even if the driving current value of motor is constant, output torque ripple and variation in the output torque due to the temperature change of motor are induced. Further the mechanical transmission efficiency of the driving system exerts an effect on the product, and there is a limit in accuracy in controlling the injection pressure (maintained pressure), so that products which do not meet the required quality standards are produced in some cases.
In order to solve the above described problems, the torque sensor, which is generally employed in the machine tool for detecting electrically the output of the motor, and the torque control apparatus which implements the closed loop are used together. However, because detection of the injection force at low cost and high accurancy has not been developed, and because the two controlling regions, one involving injection speed control and the other injection pressure control is primarily in another region certain difficulties arise which require intricate injection control, particularly in view of the use of closed loop control. Therefore, to date a suitable method for controlling the transition between the two control regions which is effective for eliminating the effect of the inertia force of motor rotor is not known. Since further method of switch timing is not fully developed, a solution to the above described problem is not available.
Although a detection method for injection of force has developed, and injection pressure control based on torque and using closed loop control is possible as is generally used in the machine tool, in addition to control problems remain injection force. The main problem relates to the inability of attaining control with with high accuracy. The reason is as follows. The load presented to the injection screw or the plunger is the melted resin. The force transmission mechanism of the motor is extremely low in stiffness, compared to the total stiffness of the system. Therefore in the transient period, a big difference develops between the set point, which generates the excessive motor output, and the measured value. Consequently, pressure vibration is induced.